Semi-conductor alloys



June 3, 195s L, w GILART 2,837,618

SEMI-JONDUCTOR ALLoYs 2 Sheelbs-Sheet 2 INVENTOR. Les W. GlLDAR-r UnitedStates Patent O SEMI-CONDUCTOR ALLoYs Lee William Gildart, Irvington, N.J., assignor to Jack Waldman, Louis Waldman, and Edward Sagerman, 1.-Irvington,N.'J.v

Application August 6, 1954, Serial No. 448,284

s claims.' (ci. 20L- 63) lt .is known that alloys' 'of antimony andselenium,

particularly an alloy having the atomic ratio of 2:3 (SbzSea), havesubstantial electrical resistivity and belong to the class of substancesknown as semi-'conduc-- tors. Thermistors are thermally sensitiveresistors made ot' semi-conductors .whose kresistance changes markedlywith small changes in the temperature. At present thermistors have wideapplication as .circuit elements in the radio and communications elds,and for other purposes.

Thermistors canYalso be used in thermometers of the electric resistancetype, using aresistive elementof the semi-conductor material in the formvof a pro'be. The semiaconductor properties of pure antimony triselenideand of this compound containing l percent of 'arsenic impurity areknownbut this compound has not Vbeen found of practical usefulnessbecause its thermo-resistive behavior is erratic and unpredictable andvaries widely from one batch of material to the next without denitelyascertainable reason or possibility of control.

It is an object of-the instant invention to provide semiconductorvmaterials, the electrical properties otwhich 'are completely predictableand easily controllable. it is aA further object of the instantinvention to provide processes for the production of new thermistormaterials.

lt is afurther object of the instant invention Ato, provide lan improvedthermometerv probe which Yis reliable and durable in use and suitablerfor a wide Yfrange of temperatures. f I p f Y, it is a furtherf objectof the instant invention to provide commercially feasible processesforfthe production ,I iprobes containingnovel semi-conductor otthermometer material. Y Y

Thesez and' othe'robjects of'the instantA invention will become moreclearly understood from the following des'criptionm 1 'i v l i it hasbeen'discoveredthatzwhen a compound selected from thegrouplrconsisting-ofrthe `trisele'nides and the trisultides yoffafmetallic' elementof the'phosp'horus family is; alloyed;;in"themannerhereinafter described, withfa compoundfselected `from the vgroupconsisting of arsenic triselenidek audli'arsenic.trisulde, a family'ofimproved semi-conducting materials are produced.' :The electricalproperties-of lthese thermistor'.materialsrare predictableand;controllableg-,andthey are suitable for use' inthermometerprobesfasihe'reinafter described,v and for other purpcsesff-f-Qln ,practicing the invention, the` 'triselenide or theatrii PatentedJune 3, 1958 as a thermistor material because its thermo-resistivebehavior was not easily reproducible. Arsenic triselenide at roomtemperature is a glass with Very high resistivity:

and is classed as an insulator. It is surprising thatin producing .analloy of these two compounds, a materialwhich is thermo-resistivelystable and predictable is ob-V tained. obs-cure, the fact is wellestablished. t.

in order to utilize with security and continuityV the semi-conductingqualities of the antimony Ytriselenide-Y arsenic triselenide alloyproduced in accordance with the instant invention, still furtherphysico-chemical characteiristics must be considered. Particularlyygoodand` permanent Contact must'be established between ,the ther#sulfidefsalt ofvveitherantimony or bismuth is alloyed with f l mistorand other solids in order to Linsure'electrical con-I tact with therequired electrodes and conductorsrand, iny many instances, also for-the'purpose of insuringgood thermal contact with the medium, thetemperature iof which is to be measured. Ithas been discoveredtha'tadequate contact can best be obtained in either /of two ways: (l) byinserting self-heated electrodes into ythe body of the thermistor allo-yitself, and (2) by casting and shrinking the thermistor alloy around theelectrode orl other solid body to be contacted. Av secure electriccontact seems to require additionally/such performance of the casting ormolding `and shrinking process as Vtoicreate a surface chemical reactionbetween the ,probe material and the electrode. f

ln practicing the instant invention, an improved probe material, analloy of yantimony triselenide andarse'nic triselenide is produced, andin this embodimentdtis important that the individual compounds should beun. contaminated by any other elements ortracesoflelernents exceeding0.1 percent by weight of the alloy. In Vaddi-V tion, in order to insurethe benets of the stability and reproducibility of the alloy as'previously mentioned, the

4actual weight ratios must Ibe kept uniform between sucdeviation fromtheoretical uniformity. y n

For example, each of the elements,y antimony andselenium, which comprisethe compound, antimonyy tri-1 selenide, should be at least about 99.9percentpureby weight, and each of the elements, arsenic :and/selenium,which comprise the compound, arsenic triselenide, should be at leastabout 99.9 percent pure by weight.' Further, when the two compounds areweighed out for alloying one with the other, the proportionsV should `beheldwithin limits of better than about 0.1 percent by weight in order toproduce a semi-conductor alloy of predictable and reproducibleelectrical properties., v

The relation between the electrical resistivity and the temperaturecoeicient of electrical resistivity of an alloy on the one han'd and theproportionsrin weight percentages of its components on the other areclearly established in the following table:

' v Electrical Temperature SbiSea `AsiSe Resistivty Ooetlcient at Y(Weight v(Weight (Ohm-Cm. at Y 25 C..(Per Percent) Percent) 25oCenticent per grade) Degree) Although the reason for this Vresult islargely lt will be noted that the electrical resistivity increasesgeometrically with the increasing As2Se3 content, and that thetemperature coefl'lcient of resistivity also increases, but much moreslowly. It is impractical to set limits as to the range of percentagesof Sb2Se3 and As2Se3 in the alloy which might be of the greatest use forsemiconductor or thermistor materials. Whether the thermistor is to bemade with low or high percentages of Sb2Se3 is a matter of choice, thefactors of which are well known to those skilled in the art, and dependsentirely on the particular application to which the material is to beput.

In other embodiments of the invention, the compounds, antimony selenideand arsenic trisulde, antimony trisulde and arsenic trisulfde, orantimony trisullide and arsenic triselenide are alloyed together in themanner described herein, and the resulting alloys have electricalresistivities and thermal coefficients similar to the alloys of antimonytriselenide and arsenic triselenide. However,

these alloys have slightly lower mechanical strength than the antimonytriselenide-arsenic triselenide alloys which are the preferredthermistor materials. 1

In the process of preparing the instant alloys, it is a feature ofparamount importance that the constituents should be pure and theirweights accurate as stated above. In practicing the invention, it isfurther important that in the melting and alloying step, excessivelyhigh temperatures and prolonged periods of heating be avoided in orderto prevent changes in the weight proportion by evaporation orsublimation. and arsenic triselenide become fluid at temperaturesslightly above about 600 C., but inasmuch as the elements Selenium andarsenic both have a tendency to sublime out of the mixture at thistemperature, it is essential that the alloying and casting or molding beperformed rapidly in orderto avoid substantial loss of these elements.it is almost impossible to avoid loss of some of these elements, butloss is insignificant when the process is carried out at a temperaturebetween about 630 and about 670 C., as hereindescribed.

On the other hand, it is less important in the instant process than itwas in prior attempts to prepare thermistors of antimony-selenium alloysthat the exact conditions of heating and cooling be precisely duplicatedbetween diterent batches for the same use. For instance, theantimony-selenium alloys exhibited majo-r irregularity of resistivity,depending on whether the cooling down, after solidification, was rapidor slow, and on other factors. In this respect the instant alloys areconsiderably steadier. Of course, major divergencies should be avoidedto insure the greatest possible uniformity of results; for instance, itis not advisable to cool one part of a batch abruptly and to slowlyanneal another. However, the antimony triselenide-arsenic triselenidealloys as described herein are substantially free from theirregularities mentioned as long as the alloy preparation ishandled inaccordance with the instant process and in the manner obvious to personsskilled in the art.

It has also been found possible to formulate and prepare a pro-beofthermistor material so as Yto simplify the calibration of a thermometercontaining the probe: this possibility arises from the fact that,subject to the precautions which have been stated, different mixtures ordifferent batches of the alloys, such as the antimonytriselenide-arsenic triselenide alloys, can Vbe rehreated and againmixed to produce intermediate alloys having desired electricalproperties and without resulting irregularity in therrnoresistivebehavior. As a result,a ner adjustment of resistivity becomes possible,by-remixing two or more alloys in-which the proportions of antimonytriselenide to arsenic triselenide are Vnot exactly the same. No suchmixing is successfully practiced with other semiconductors or thermistormaterials.

It will be understood by persons skilled in the art that every batchshould be calibrated, for precision work.

The antimony triselenide l However such calibration is greatlyfacilitated by the possibility of relatively accurate pre-selection ofthe Weight of the components in the alloying process and of mixingbatches, as described. It is believed that no other material ispresently known which provides such versatility together with suchaccuracy of pre-selection.

Actual thermometers produced in accordance with this invention can bestbe described in conjunction with the drawings appended hereto, wherein:

Figure l is an enlarged longitudinal section through a rst type ofthermometer probe produced in accordance herewith.

Figure 2 is a section through the probe of Figure l, the section beingtaken along lines 2 2 in Figure 1.

Figure 3 is a View generally similar to Figure l but showing a modiliedprobe.

Figure 4 is a view generally similar to Figure 2 and showing a sectionthrough the probe of Figure 3 along the lines 4-4 in Figure 3.

Figure 5 is a view generally similar to Figure l but showing a secondmodification.

Figure 6 is a view generally similar to Figure 2, taken along the lines6-6 in Figure 5.

Figure 7 is a view generally similar to Figure l but showing a thirdmodification Figure 8 is a view generally similar to Figure 2, takenalong lines 8-8 in Figure 7.

Figure 9 is still another view generally similar to Figure l and showinga fourth modification.

Figure l0 is a view generally similar to Figure 2 taken along lines10--10 in Figure 9.

In all views the numeral 11 indicates the temperature sensitive probe,slug or cartridge. Preferably this element is made from the antimonytriselenide-arsenic triselenide Sb2Se3-As2Se3 alloy described above, andparticularly from a suitably predetermined batch, for such resistivityas may be suitable for theelectric system to be used, wherein thethermometer forms a variable resistance element. However, certainaspects of the thermometer probe design to be described presently areapplicable also to other thermistor materials.

Referring first to Figures l and 2: the Sb2Se3-As2Se3 probe 11,preferably containing less than 30.00 percent `by weight of As2Se3, isdisposed within a nickel-silver shell 12 at the closed tip thereof. Thisshell serves as one of the electrodes in the circuit containing theprobe as a variable resistor; the other electrode 14 being anickel-silver, Kovar, rodar or Nichrome wire inserted in the centralupper part of the probe 11. Kovar and rodar are alloys containingnickel, cobalt, manganese and iron. In order to avoid inadvertentgrounding of the outer electrode 12, this electrode may be provided withan insulating envelope or shield 15 which may consist in a plasticcoating, a removable glass envelope or the like, depending upon theservice to which the outer thermometer surface is subjected. l

Attention is drawn to the fact that, while the nickelsilver electrode 12surrounds and mechanically protects the bulk of the probe 11, a centralprojection or finger 16, formed integrally or otherwise in or on theinside of the tip 13, is surrounded by the probe material, as is the endor tip 17 of the electrode wire 14. This arrangement is important forthe production and maintenance of contact. v

In the process of producing the thermometer probe, the suitably selectedmaterial for the probe, for example, an antimony triselenide-arsenictriselenide alloy, is filled into the tip 13, in powdered or granulatedcondition, the wire 14 being'held in position in well known manner. Thetip is then heated in order to remelt and re-fuse the probe material,subject to substantially the same precautions as have been mentionedabove in connection with the original fusion process. Thereupon thecasting or molding of the probe is completed by cooling it down tonormal temperatures, incident to which the differential shrinkage vofthepobeniaterial co'r'npared* with the metal of the electrodes causes'tight-'engagementand'lsqueezing ofthetips16and17. Y Y .y

More particularly,l it has beenA found preferable to provide some littletime delay in one ofthe later stages of this cooling process, althoughthe melting temperature, for reasons stated above, should bemaintained-onlyfor the shortest possible time, requiredfor-completemelting. At some suitable temperatures below 600 C., fornstance at approximately 550 C., it vis desirable to provide astabilization period, lfor example between about 5 and about 60 minutes,preferablyI of `about. 5- minutes. A more durable contact is thusprovided-probably due to the formation of metal selenides on Vthe'electrode instance for clinical purposes, in which event the probematerial, desirably, is prepared 4for high sensitivity in the usualclinical temperature range, that is, in the vicinity of about 100 F. Inthis connection it will beunderstood by persons skilled in the art thatsemi-conductors of different formulations have maximum temperaturec'oecient and sensitivity in one particular part oftheir basictemperature range. Furthermore, this thermometer probe will'best beformulated for such resistance-'as allows the use of weak, clinicallyinoiensive electric currents, so that in'suitableY cases the metal shell12, 13

can Ibe contacted with the body of the patientv direct, without anyelectric insulation andV therefore without heat insulation. The use ofav stainlessV steelvshell is also desirable, from the clinicalviewpoint, because the outside of such a shell can be sterilized in anautoclave. This cannot be done Ywith an ordinary fever thermometer.Themnew` VprbefmaterialV isfnot injured and not over expanded byautoclave steam temperatures ranging suitably upwards of 220 F. g

lFor other applications, itmay, be preferable ,to use higher voltagesand to formulate the probe 11 accordingly, in order to simplify thecircuit system or for other reasons. In such cases it is usuallydesirable to utilize an insulating sheath of glass or the like, in placeof the stainless steel shell 15, in the actual use of the thermometer.

In the modied 'form of Figures 3 and 4, the probe 31 is directlyenclosed in a shell 32 which has electric insulating' properties andwhich may, for instance, consist of Pyrex glass; the thickness of theshell being suitably selected for the desired Vsensitivity of thethermometer. In this event an outer shell, stainless steel or details oflthe'fY thermometer I seal arid Y electrode construe? tion,-in the upperpart remote from the probe, are not shown herein, being well known topersons' skilled in the art.

Figures 5 and 6Ashow a particularly simple or cartridge type ofapplication, `suitable mainly for the application of relatively smallthermometer elements forming permanent parts of flow systems or thelike. Here the probe 51, preferably made of the `above described alloyand also preferably cast around the electrode'ends or tips 52, 53, oropposite one another,- has a generally cylin` drical formv land issuitably inserted in a protective sleeve 54 of Pyrex, ceramic material,stainless steel, or the like. In the event'that a conductor such asstainless `steel 4is used, of course it is necessary either to insulatethe inside thereof or to make the radial distance from the tips 52, 53to the shell much greater than the longitudinal distance betweenV saidtips. Even then a certain shunting effect of the shell will exist, whichhowever need-not interfere with proper thermometric measurement if thecircuit system is properly designed in manners known to the ,art. Theends lof the cartridge-mayA be closed by plugs 55, 56 of wax or cementor other suitable sealing material, insulating theelectrode wires fromthe outsideV Vsuch current. Again the probe. is adapted to engage theelectrode firmly, in the casting and cooling process. For this purposeeach electrode slugin'this case has, on its probe-engaging surface, apreferably annular ridge Y or bead 74,2the-` outside of which is firmly:compressed .as

the probe material shrinks aroundV the same; A wafer or button 71, 7273is formed of the probe and the two electrodes, whichcan be inserted Ainthe working end of a thermometer 75 in a mannerbasically known to theart, providing excellent electrical as well as thermal contact. Thethermometer may have, vfor instance, an

outer shell 7d of stainless steel, brass or the'like,. acting p liber,plastic or the like, 79, tosseparate the inner conthe like, not shown,may be used in order to protect the thermometer from breakage when it isnot in actual use. Two parallel and similar electrodes 33 and 34 areusedin this case, both being inserted in and clamped by the upper part ofthe probe material.

In both forms, Figures l and 2 as well as 3 and 4, the interior space 13of the thermometer shell 12 or 32, above the probe (and 'also around thesame, in View of the shrinkage of the probe material, incident to themanufacture of the thermometer) is desirably lled with an inert gas suchas nitrogen or argon or the like, at such pressure or rarefaction as maybe suitable for the temperature range contemplated. The life expectancyof the probe and particularly of its contact surfaces engaging theelectrodes is improved by such use of neutral gases; the presence ofoxygen in'this space may tend to produce chemical changes in the probematerial. The

ductor 77, 78 and wafer 71,72, 73 from the outerV `terial 96, such aslead foil, may be interposed between the elements 94, 95. In this mannerelectric'contact may be lprovided by a press fit rather than theAcasting shrinkage and selenide formation used in theother examples.Furthermore the slug and electrode canV thus be made .separable andreplaceable, for instance for laboratory use and the like, whereindifferently formulatedY alloy probes may be successively clamped betweenthe same set of electrodes. v

In order to give additional understanding of the instant invention, thefollowing examples are included; however it is to be understood that'they are illustrative only and the'invention is not to be understood aslimited thereto. l Example I A thermistor material was produced byalloying anti# mony triselenide of at least 99.9% purity with arsenictlSelenide of 99.9%,purity by heating at 5a' temperature Here howeverthe probe slugY 91 is connected with 7 of about 650 C. The weight ratiowas aboutVV 96.00%. of the antimony triselenide to about 4.00% of thearsenic triselenide. The resulting alloy was cooled. The resultingmaterial had a resistivity of about 140 ohm-centimeters at 25 C. and atemperature coeicient of approximately 3.5% per Ydegree centigrade.

Example II i A thermistor material was prepared in the manner describedin Example l, except that the weight ratio was about 92.00% antimonytriselenide to about 8.00% oi arsenic triselenide. At 25 C. theresistivity or this thermistor was about 370 ohm-centimeters, and thetemperature coefficient was approximately 3.8% per degree centigrade.

Example HI A thermistor alloy was produced by remelting a portion of thealloys obtained in Example I and in Example II and mixing them in aratio by weight of 1:1. The resulting semi-conductor had a resistivityof about 225 ohm-centimeters at 25 C., and the temperature coefcient wasapproximately 3.65% per degree-Vcentigrade.

Example` IV A thermistor alloyfwas prepared in the manner described inExample I in which about 75.00% by Weight of antimony and about 25.00%by weight of arsenic triselenide were alloyed and cooled. The resultingmaterial had an electrical resistivity of about 12,000 ohmcentimeters at25 C., and `hada temperature coefficient of approximately 4.6%V perdegree centigrade.

Having thus fully described andillustrated the character of the instantinvention, what is desired to be protected and secured by Letters Patentis:

1. A semi-conductor alloy suitable for use in thermistors consisting ofa compound selected from the group consisting of the triselenides andthe trisuldes of the group consisting of antimony and bismuth, -alloyedwith at least 'about` 2% by weight of a compound selected from the groupconsisting of arsenic triselenide and arsenic trisulfide.

2. A semi-conductor alloy suitable for use in thermometer probesconsisting of a major amount of acompound selected from the groupconsisting vof the triselenides and the trisuldes of the groupconsisting of antimony tft and bismuth, and at least 2% by weight ofacompound selected from the group consisting of arsenic triselenide andarsenic trisulde. j

3. A `semi-conductor alloy suitable for use in thermistcrs kconsistingof anV alloy of antimony triselenide and atleast aboutY 2%Y by weight'ofarsenic triselenide.

, 4. A thermometer probe material comprising an alloy containing betweenabout 70% jand about 95% by weight of antimony triselenide andbetweenabout 30% and aboutSZ/o by Weight ofarsenic triselenide.

5. In a process for the production of thermistors including productionof semi-conductor material and form-- ingV a `v resistance elementtherefrom, the improvement comprising alloying substantially purearsenic triselenide and substantially pure antimony triselenide at atemperature between 630 and about 670 C., and cooling Vtheresultingthermistor material.. Y

6. A method `offproducing a 4thermometerprobe comprising melting ,abodyof an alloy containing between about;;70% and about 95% by weight ofantimony triselenide and between about 30% and about 5% by weight ofarsenic triselenide around a pair of electrode tips at atemperature'between about 630 and about 670 C., cooling the body to atemperature between about 550 and about 600 C., allowing the partiallycooled material to stand Yat such temperature for between about 5 andabout'60 minutes, and then cooling theA resulting material to aboutatmospheric temperature.

7. A thermometer probe comprisingA a solid body of,

an antimony selenium arsenic alloy cast around and thereby shrunk upon apair of electrodes, wherein one. of ,the electrodes castvinto the probebody substantially. consists in an inward projection of a metallic bodyalso forming an outer shell for the probe.

8. A thermometer probe as described in claim 7, wherein the outerVshell'rand inward projection thereofv substantially consists ofstainless steel.

' OTHER REFERENCES Cullity et al., Trans. A. L. M. E. V. 188, JanuaryV1950, Journal of Metals-pages 47-52.

1. A SEMI-CONDUCTOR ALLOY SUITABLE FOR USE IN THERMISTORS CONSISTING OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF THE TRISELENIDES AND THE TRISULFIDES OF THE GROUP CONSISTING OF ANTIMONY AND BISMUTH, ALLOYED WITH AT LEAST ABOUT 2% BY WEIGHT OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ARSENIC TRISELENIDE AND ARSENIC TRISULFIDE. 